Numerical study of the October 2000 torrential precipitation event over eastern Spain: Analysis of the synoptic-scale stationarity

Abstract

A torrential precipitation event affected eastern Spain during 21 to 24 October 2000. Total accumulated rainfall higher than 500 mm was registered at some locations, with values up to 300 mm in a 24-h period. The synoptic-scale charts for these days show the presence of a cold cutoff low aloft, south of the Iberian Peninsula, as a part of an Ω-blocking structure over Europe. At low levels, persistent easterly winds, established between a dominant anticyclone over eastern Europe and a cyclone over Morocco, are found over the western Mediterranean throughout the entire period. Satellite images show the advance and breaking away of a trough, with an associated cold front, over the Iberian Peninsula, which resulted in the cutoff low formation. Later, scattered convective cells are detected along the eastern Spanish coast during more than 3 days. Numerical simulations reveal that the convective environment was developed by the low-level advection of warm and moist air from central Mediterranean, being charged of moisture by evaporation from the sea. Sensitivity runs confirm that the synoptic-scale persistent low-level easterly flow, and specifically, the presence of a low level jet, was crucial for the subsynoptic processes leading to the long lasting torrential rainfall over the exposed terrains of eastern Spain. The stagnancy of the low-level flow is attributed to the quasi-stationary characteristics of the upper level cutoff low located south of the Iberian Peninsula. Experiments with modified upper-level potential vorticity distributions reveal that slight deviations from the observed configuration result in enhanced mobility of the low-level flow pattern, and thereby a reduction of the precipitation persistence. This suggests the major importance of a correct representation in the model initial conditions of the intensity and location of the upper level features, in order to obtain valuable numerical forecasts of these heavy rainfall events.